1. Field of the Invention
The present invention relates to a DC-DC converter for converting an input voltage into a predetermined output voltage by executing at least one of a plurality of kinds of PWM control, a DC-DC converter control apparatus for controlling such DC-DC converter, a power supply apparatus and an electronic equipment which convert voltage by such DC-DC converter or DC-DC converter control apparatus, and a control method for DC-DC converter by above mentioned method.
2. Description of Related Art
A portable electronic equipment such as a mobile phone or a notebook-sized personal computer uses battery as power source. However, since the output of battery undergoes a change such as output decrease due to electric discharge, a portable electronic equipment comprises a DC-DC converter for converting an output voltage of the battery into a voltage to be used in the equipment (see Japanese Patent Application Laid-Open No. 10-225108 (1998), for example). Known methods for converting voltage are of three kinds: a step-down method, a step-up method and a step-up/down method. In a step-down method, battery having an output voltage higher than the voltage to be used in the equipment is used and the output voltage is stepped down by a step-down DC-DC converter to the voltage to be used in the equipment. With this step-down method, though only voltage equal to or lower than the output voltage of the battery can be outputted, extremely high conversion efficiency such as 90% to 95% can be realized.
In a step-up method, battery having an output voltage lower than the voltage to be used in the equipment is used and the output voltage is stepped up by a step-up DC-DC converter to the voltage to be used in the equipment. With this step-up method, though voltage equal to or higher than the output voltage of the battery can be outputted, the conversion efficiency is not higher than 80% to 88%. A step-up/down method is used for a case where the output voltage of the battery rises above and falls below the voltage to be used in the equipment, wherein the output voltage of the battery is stepped up or stepped down by a step-up/down DC-DC converter to the voltage to be used in the equipment when the output voltage is lower or higher than the voltage to be used in the equipment.
FIG. 1 is a schematic circuit diagram showing an example of the configuration of a conventional flyback step-up/down DC-DC converter which uses a transformer. An input voltage terminal Vin is connected with an input terminal (drain) of an n-type field-effect transistor 1 (which will be hereinafter referred to as an FET 1) as a switching circuit, an input terminal of primary winding L1 of a transformer T is connected with an output terminal (source) of the FET 1, and an output terminal of the primary winding L1 is connected with an earthing terminal. Moreover, a control terminal (gate) of the FET 1 is connected with an output terminal DH1 of a control unit 2 and is turned ON/OFF by the control unit 2.
An input terminal of secondary winding L2 of the transformer T is connected with an earthing terminal and an input terminal (source) of an FET 4 as a synchronous rectification circuit is connected with an output terminal of the secondary winding L2. An output terminal (drain) of the FET 4 is connected with an output voltage terminal Vout of the DC-DC converter, and a control terminal (gate) of the FET 4 is connected with an output terminal DL1 of the control unit 2 and is turned ON/OFF by the control unit 2. Here, the output terminal DL1 outputs a signal *Q which is obtained by inverting an output signal Q of the output terminal DH1.
The output voltage terminal Vout of the DC-DC converter is connected with an earthing terminal via a smoothing capacitor C1 for smoothing and with an FB terminal of the control unit 2. The FB terminal of the control unit 2 is connected with an earthing terminal via a series circuit of a resistor R1 and a resistor R2. The node between the resistor R1 and the resistor R2 is connected with inverting input of an error amplifier ERA. Moreover, a reference voltage source e1 is connected with noninverting input of the error amplifier ERA. Output of the error amplifier ERA is connected with noninverting input of a comparator PWM for PWM control and an oscillator OSC for outputting triangular wave is connected with inverting input of the comparator PWM.
The control unit 2 compares a voltage, which is obtained by dividing the output voltage Vout of the DC-DC converter by the resistors R1 and R2, with a reference voltage e1 and outputs a voltage corresponding to the difference from the error amplifier ERA. The comparator PWM compares the output voltage of the error amplifier ERA with an output voltage of the oscillator OSC and outputs an ON signal when the output voltage of the error amplifier ERA is higher than the output voltage of the oscillator OSC. Accordingly, the pulse width of the output signal of the comparator PWM increases or decreases according to the output voltage of the error amplifier ERA.
The output Q of the comparator PWM is given to the FET 1 while the inverted output *Q is given to the FET 4. Accordingly, the FET 1 is turned on and the FET 4 is turned off when the comparator PWM outputs an ON signal. In contrast, the FET 1 is turned off and the FET 4 is turned on when the comparator PWM outputs an OFF signal (does not output an ON signal). When the FET is on, since an input voltage Vin is applied to the primary winding L1 of the transformer T, the electric current which flows through the primary winding L1 increases. Here, since the FET 4 is off, no electric current flows through the secondary winding L2 of the transformer T and energy is stored in the primary winding L1 of the transformer T. Then, when the FET 1 is turned off and the FET 4 is turned on, the energy stored in the primary winding L1 of the transformer T is discharged from the secondary winding L2 to the smoothing capacitor C1.
As described above, energy is stored in the primary winding L1 of the transformer T when the FET 1 is on (Ton) and the energy stored in the primary winding L1 is discharged from the secondary winding L2 when the FET 1 is off (Toff). Assuming that the winding ratio between the primary winding L1 and the secondary winding L2 is 1:1, the output voltage Vout is expressed by the following equation.Vout=(Ton/Toff)×Vin
Accordingly, by changing the ON/OFF ratio of the FET 1, the output voltage Vout can be larger, and also smaller, than the input voltage Vin. However, since the coil L1 for storing energy is different from the coil L2 for discharging the energy, there is a problem that the voltage conversion efficiency depends on, for example, the degree of coupling between the coils.
On the other hand, shown in FIG. 2 is a conventional step-up/down DC-DC converter which uses the same coil as a coil for storing energy and as a coil for discharging the energy. In FIG. 2, an FET 3 as a switching circuit and an FET 2 as a synchronous rectification circuit are added in order to replacing the transformer T with a choke coil L1 and to share the primary winding L1 and the secondary winding L2 of the transformer T. An input terminal (drain) of the FET 3 is connected with an output terminal of the choke coil L1 and an output terminal (source) of the FET 3 is connected with an earthing terminal. Being connected with the output terminal DH1 of the control unit 2, a control terminal (gate) of the FET 3 is turned ON/OFF by the control unit 2 simultaneously with the FET 1. Moreover, an input terminal (drain) of the FET 2 is connected with an input terminal of the choke coil L1 and an output terminal (source) of the FET 2 is connected with an earthing terminal. Being connected with the output terminal DL1 of the control unit 2, a control terminal (gate) of the FET 2 is turned ON/OFF by the control unit 2 simultaneously with the FET 4.
When the FET 1 and the FET 3 are turned on, the FET 4 and the FET 2 are off and the input terminal of the choke coil L1 provides an input voltage Vin, so that the input voltage Vin is applied to the choke coil L1 and the electric current which flows through the choke coil L1 increases. Then, when the FET 1 and the FET 3 are turned off and the FET 4 and the FET 2 are turned on, the output terminal of the choke coil L1 is connected with the output terminal Vout of the DC-DC converter, so that the energy stored in the choke coil L1 is discharged to the smoothing capacitor C1.
With the DC-DC converter shown in FIG. 2, since the same coil is used as a coil for storing energy and as a coil for discharging the energy, the efficiency of the DC-DC converter does not depends on, for example, the degree of coupling between the coils of the primary winding L1 and the secondary winding L2 of the transformer T shown in FIG. 1. With the DC-DC converter shown in FIG. 2, however, since a total of four switches—the FET 1 and the FET 3, and the FET 4 and the FET 2—are turned ON/OFF, that is, since twice as many switches as those of the DC-DC converter shown in FIG. 1 are turned ON/OFF simultaneously, the efficiency of switching drive is lowered.
Here, in the control unit 2 of the DC-DC converter shown in FIG. 2, the FET 1, the FET 2 and the choke coil L1 compose a step-down DC-DC converter. Moreover, the choke coil L1, the FET 3 and the FET 4 compose a step-up DC-DC converter. That is, the DC-DC converter shown in FIG. 2 is a step-up/down DC-DC converter composed of series-connected step-down DC-DC converter and step-up DC-DC converter sharing the choke coil L1, and can be rewritten as in the circuit diagram of FIG. 3.
In FIG. 3, a step-down comparator PWMD is a comparator for executing PWM control for step-down operations, and executes ON/OFF control of the FET 1 and the FET 2. Similarly, a step-up comparator PWMU is a comparator for executing PWM control for step-up operations, and executes ON/OFF control of the FET 3 and the FET 4. Moreover, an offset voltage source e2 is connected between inverting input of the step-up comparator PWMU and the oscillator OSC. Other configuration is the same as those of FIG. 2. One comparator PWM executes control in the DC-DC converter shown in FIG. 2 while a step-down comparator PWMD and a step-up comparator PWMU are provided separately in the DC-DC converter shown in FIG. 3 for relatively executing PWM control with separate duties.
The relation between the input voltage Vin and the output voltage Vout of the step-down DC-DC converter is represented by the following equations.Vout/Vin=Ton/(Ton+Toff)Vout=Vin×Ton/(Ton+Toff)
Here, assuming that the on-duty of step-down PWM control is d1, the following equation is provided.Vout=Vin×d1
Moreover, the relation between the input voltage Vin and the output voltage Vout of the step-up DC-DC converter is represented by the following equations.Vout/Vin=(Ton+Toff)/ToffVout=Vin×(Ton+Toff)/Toff
Accordingly, assuming that the on-duty of step-up PWM control is d2, the following equation is provided.Vout=Vin×1/(1−d2)
Accordingly, the relation between the input voltage Vin and the output voltage Vout of the step-up/down DC-DC converter composed of series-connected step-down DC-DC converter and step-up DC-DC converter is represented by the following equation.Vout=Vin×d1/(1−d2)
When the input voltage Vin is higher than the output voltage Vout and the step-up DC-DC converter is under suspension, the on-duty d2 of step-up PWM control is equal to zero and the above equation is rewritten as in the following equation.Vout=Vin×d1
Accordingly, in this case, step-down is achieved by step-down PWM control. Moreover, when the input voltage Vin is lower than the output voltage Vout and the step-down DC-DC converter has on-duty of 100%, the on-duty d1 of step-down PWM control is equal to 1 and the above equation is rewritten as in the following equation.Vout=Vin×1/(1−d2)
Accordingly, in this case, step-up is achieved by step-up PWM control.
FIG. 4 is a timing chart schematically showing an example of the operational state of a case where step-up PWM control is not executed and step-down PWM control is executed. In FIG. 4, triangular wave inputted into the step-up comparator PWMU is shown in broken line and triangular wave inputted into the step-down comparator PWMD is shown in solid line. It should be noted that offset by the offset voltage source e2 arises at the triangular wave (broken line) inputted into the step-up comparator PWMU. Moreover, the output of the error amplifier ERA is included in the step-down operation area (amplitude area of triangular wave in solid line) but is not included in the step-up operation area (amplitude area of triangular wave in broken line). Regarding the step-down comparator PWMD, the FET 1 is turned on and the FET 2 is turned off when the triangular wave (solid line) is lower than the output of the error amplifier ERA, and the FET 1 is turned off and the FET 2 is turned on when the triangular wave (solid line) is higher than the output of the error amplifier ERA. On the other hand, regarding the step-up comparator PWMU, since the triangular wave (broken line) is higher than the output of the error amplifier ERA and the two never meets, the duty of step-up PWM control becomes 0%, the FET 3 remains off and the FET 4 remains on.
FIG. 5 is a timing chart schematically showing an example of the operational state of a case where step-down PWM control is not executed and step-up PWM control is executed. Each waveform in FIG. 5 is the same as that in FIG. 4. In FIG. 5, the output of the error amplifier ERA is included in the step-up operation area but is not included in the step-down operation area. Regarding the step-up comparator PWMU, the FET 3 is turned on and the FET 4 is turned off when the triangular wave (broken line) is lower than the output of the error amplifier ERA, and the FET 3 is turned off and the FET 4 is turned on when the triangular wave (broken line) is higher than the output of the error amplifier ERA. On the other hand, regarding the step-down comparator PWMD, since the triangular wave (solid line) is lower than the output of the error amplifier ERA and the two never meets, the duty of step-down PWM control becomes 100%, the FET 1 remains on and the FET 2 remains off.
FIG. 6 is a timing chart schematically showing the operational state of a case where both of step-up PWM control and step-down PWM control are executed. Each waveform in FIG. 6 is the same as that in FIG. 4 and FIG. 5. In FIG. 6, the output of the error amplifier ERA is included in both of the step-up operation area and the step-down operation area. Regarding the step-up comparator PWMU, the FET 3 is turned on and the FET 4 is turned off when the triangular wave (broken line) is lower than the output of the error amplifier ERA, and the FET 3 is turned off and the FET 4 is turned on when the triangular wave (broken line) is higher than the output of the error amplifier ERA. Moreover, regarding the step-down comparator PWMD, the FET 1 is turned on and the FET 2 is turned off when the triangular wave (solid line) is lower than the output of the error amplifier ERA, and the FET 1 is turned off and the FET 2 is turned on when the triangular wave (solid line) is higher than the output of the error amplifier ERA. Here, the ON/OFF switching timing for step-down PWM control and the ON/OFF switching timing for step-up PWM control are out of synchronization.
For executing only one of step-up PWM control and step-down PWM control, one pair of switches are turned ON/OFF. On the other hand, for executing both of step-up PWM control and step-down PWM control, two pairs of switches are turned ON/OFF, so that loss on ON/OFF switching of the switches is doubled. Moreover, when two pairs of switches are turned ON/OFF, since the switches are turned ON/OFF separately (asynchronously) for each pair, there is a problem that the loss is further increased. Here, the loss on ON/OFF operations of the switches is composed of drive loss of the switches; and resistance loss which arises in the transient area when the switches are switched from off to on or from on to off.